1. Field of the Invention
The present invention relates to a motor suitable for a reduction in size and optical apparatus using the motor.
2. Related Background Art
FIG. 16 is a sectional view showing an arrangement of a conventional stepping motor. FIG. 17 is a partial sectional view simply illustrating magnetic fluxes flowing from the stator of the stepping motor in FIG. 16.
Referring to FIG. 16, two bobbins 101 having stator coils 105 wound therearound are juxtaposed in the axial direction of a rotor 109. These two bobbins 101 are respectively fixed to different stator yokes 106. Stator teeth 106a and 106b are alternately formed on the inner diameter surface of each stator yoke 106. The stator yokes 106 are respectively fixed to cases 103. Two stators 102 respectively corresponding to the two excitation stator coils 105 are constituted by these members.
A flange 115 and bearing 108 are fixed to one of the two cases 103, and the other bearing 108 is fixed to the other case 103. A rotor 109 is formed from a rotor magnet 111 fixed to a rotor shaft 110. The rotor shaft 110 is rotatably supported by the two bearings 108.
In the stepping motor described above, however, since the cases 103, bobbins 101, stator coils 105, and stator yokes 106 are arranged on the outer surface of the rotor 109, the outer dimension of the motor becomes large. In addition, the magnetic fluxes which are generated when the stator coils 105 are energized mainly pass through an end face 106a1 of the stator tooth 106a and an end face 106b1 of the stator tooth 106b, as shown in FIG. 17. For this reason, much of magnetic flux does not act on the rotor magnet 111, and hence the motor output does not increase much.
In order to increase the output of a motor while decreasing its size, a motor having an arrangement like the one disclosed in U.S. Pat. No. 5,831,356 has been proposed. In this motor, a rotor is formed by dividing a cylindrical permanent magnet into equal portions in the circumferential direction and alternately magnetizing the portions to different poles, and the first coil, rotor, and second coil are sequentially arranged in the axial direction of the rotor (the axial direction of the motor). The first outer magnetic poles and first inner magnetic poles which are excited by the first coil are arranged to face the outer and inner surfaces of one half portion of the rotor in the axial direction, whereas the second outer magnetic poles and second inner magnetic poles which are excited by the second coil are arranged to face the outer and inner surfaces of the other half portion of the rotor in the axial direction.
FIG. 18 is a sectional view of a motor having the above arrangement. This motor includes a magnet 311, a first coil 312, a second coil 313, a first stator 314, first outer magnetic poles 314a and 314b, first inner magnetic poles 314c and 314d, a second stator 315, second outer magnetic poles 315a and 315b, second inner magnetic poles 315c and 315d, a coupling ring 316 for holding the first stator 314 and second stator 315, and an output shaft 317 to which the magnet 311 is fixed and which rotates together with the magnet 311. This output shaft 317 is rotatably supported by bearing portions 314e and 315e of the first and second stators 314 and 315.
According to this arrangement, a motor having a high output level and a small diameter can be provided.
Unlike such a motor having a reduced diameter, a motor having a reduced length in the axial direction is disclosed in Japanese Patent Application Laid-Open No. 2000-50601. FIG. 19 shows this motor. The motor is constituted by a plurality of coils 301, 302, and 303 and a disk-like magnet 304. As shown in FIG. 19, each coil has a thin coin-like shape, whose axis is parallel to the axis of the magnet. The disk-like magnet is magnetized in the axial direction of the disk-like shape, and the magnetized surface of the magnet faces the axis of the coil.
According to this arrangement, as indicated by the arrows in FIG. 20, a large number of magnetic fluxes generated from the coils do not pass through the magnet. The output level is therefore not high for the size of the motor.
There seems to be room for improvement in providing a motor having a reduced length in the axial direction.
According to an aspect of the present invention, there is provided a motor comprising a rotor having a cylindrical magnet having an outer surface divided into portions in a circumferential direction which are alternately magnetized to different poles, a first outer magnetic pole which is excited by a first coil and faces the outer surface of the magnet within a first predetermined angular range, a first inner magnetic pole which is excited by the first coil and faces an inner surface of the magnet, a second outer magnetic pole which is excited by a second coil and faces the outer surface of the magnet within a second predetermined angular range, and a second inner magnetic pole which is excited by the second coil and faces the inner surface of the magnet, wherein the first and second outer magnetic poles are positioned on the same circumference centered on the magnet.
According to another aspect of the present invention, there is provided a motor comprising a rotatable rotor having a cylindrical magnet having an outer surface alternately magnetized to different poles in a circumferential, a first outer magnetic pole which is excited by a first coil and faces the outer surface of the magnet, a second outer magnetic pole which is excited by a second coil and faces the outer surface of the magnet, and an inner magnetic pole which faces an inner surface of the magnet, wherein the first and second outer magnetic poles are positioned on the same circumference centered on the magnet, and when the center of one outer magnetic pole faces the center of a region of the magnet, the center of the other outer magnetic pole faces a boundary portion between regions of the magnet.
In addition, there are provided a photographing apparatus which translates a lens holder by using this motor as a driving source and a photographing apparatus which opens shutter blades by using the motor as a driving source.